1. Field of the Invention
The present invention relates to surface-emitting-type semiconductor lasers that emit laser beams perpendicularly to semiconductive substrates.
2. Related Art
Semiconductor lasers are classified into edge-emitting-type semiconductor lasers that emit laser beams from cleavage planes and surface-emitting-type semiconductor lasers that emit laser beams perpendicularly to semiconductive substrates. In semiconductor lasers, an active layer having a gain with respect to light is disposed in a resonator provided between mirrors so that light moving back and forth in the resonator is amplified until oscillation occurs. Edge-emitting-type semiconductor lasers have been widely used since they have simplified configurations having mirrors as cleavage planes and can produce high-output laser beams. On the other hand, surface-emitting-type semiconductor lasers have semiconductive multilayered mirrors or dielectric mirrors. Thus, they have complicated configurations; however, they have the following advantages: (1) low threshold currents, (2) ability to be arrayed on semiconductive substrates, (3) a single mode of longitudinal oscillation, (4) stable oscillation wavelength, and (5) circular (conical) beams being obtainable.
Surface-emitting-type semiconductor lasers, however, have a problem of difficult control of polarization planes. Since an edge-emitting-type semiconductor laser has a resonator consisting of a waveguide, TE waves have larger reflectance compared with TM waves on the end faces of the waveguide, and thus the TE waves having an electric field vector parallel to a semiconductive substrate will oscillate. Light emerging from the edge-emitting-type semiconductor laser has a stable polarization plane without fluctuation. In contrast, in a surface-emitting-type semiconductor laser, it is difficult to enhance the reflectance of mirrors with respect to light polarized in a specific direction and to raise the gain of the activation layer. Since the surface-emitting-type semiconductor laser has an isotropic configuration with respect to polarized light, it has problems such as fluctuation and instability of the polarization direction. Reflectance of most beam splitters and diffraction gratings depends on the polarization direction, hence fluctuation of the polarization direction hinders use of the semiconductor laser when it is mounted in an optical apparatus. Furthermore, an unstable polarization plane causes irregular movement of the orthogonal polarization directions on the polarization plane, which will generate noise.
The following are countermeasures for solving this problem. In a first method, thin metallic lines are arranged in one direction on a semiconductive multilayered mirror to increase the reflectance of the mirror with respect to the polarized light in a specific direction. Since the reflectance increases with respect to light having a polarization direction parallel to the thin metallic lines, this method is effective to some extent for stabilization of the polarization plane; however, the thin metallic lines must have a size which is less than the wavelength of light; hence it is difficult to form them. An alternative method uses dependence of the gain on crystal orientation, in which an active layer is formed on a high-index oriented crystal plane, such as a (311)A plane or a (311)B plane, to raise the gain of the active layer with respect to polarized light in a predetermined direction. This method, however, has disadvantages, such as difficulty in growing crystals and in obtaining high output.
In addition, an attempted method is control of polarized light by a resonator having a specified shape. It has been found that a rectangular resonator facilitates orientation of polarized light in the short or long side direction, and this fact suggests possibility of ready control of the polarized light. Its cause, however, has not been clarified, and the method has less reproducibility.